In the process of xerography, a light image corresponding to the original to be copied is typically recorded in the form of a latent electrostatic image upon a photoconductive member. This latent image is developed, that is to say, made visible, by the application of a pigmented thermoplastic resin, commonly referred to as toner. The thus developed latent image is thereafter transferred from the photoconductive member onto a copy sheet, such as for example, paper. The copy sheet is subsequently passed to a fusing apparatus which affixes the image onto the sheet and is later discharged from the machine as a final copy.
One approach to fixing the toner particles onto the copy sheet has been to pass the copy sheet with toner images thereon through a fusing nip formed by a heated fuser roll and a backup roll. As it passes therethrough, the copy sheet is simultaneously pressed and heated so that the toner becomes softened and firmly attached to the copy sheet.
In such a fusing arrangement, opening and closing of the fuser nip is commonly controlled by a cam rotatable in synchronization with movement of the copy sheets. Frequently therefore, during the processing of variable length sheets (i.e., sheets of different length than the selected standard which is the longest anticipated sheet length), the rollers are allowed to remain in contact during periods in which no copy sheet is disposed therebetween. This prolonged direct contact commonly results in the overheating of the backup roll. Such overheating of the roller may result in a paper jam as the copy sheet will tend to follow the backup roll rather than continuing along the intended paper path beyond the fuser station. This backup roll sticking problem is especially aggravated during the fusing of duplex copies (i.e., sheets with toner copy on both sides) due to the cohesive nature of the toner. Overheating of the roller surface may also result in a phenomena referred to in the printing art as "offset" wherein toner adheres to the roller surface and is transferred to the same or next copy sheet.
Substantial damage may also be caused to the rollers from entrapment of copy sheets between the heated fuser roll and the backup roll when there is a loss of power during a copying operation. It is desirable therefore that the fuser rollers remain in an open position during standby mode and similarly during a copying operation that the fusing nip be closed only when there is paper between the rollers. Thus, it is desirable that the fusing nip be opened during the intersheet gap that may exist between adjacent copy sheets and be closed only when the next sheet arrives. Likewise, when a paper jam is detected, control of the closure mechanism is desirable so that fuser roller opening may be effected and damage to the rollers avoided. Further, it is desirable to open the fuser nip to facilitate maintenance. IBM TDB "Fuser Configuration" (5/81, pp. 5622-5623) describes the details of a pivoting and latching mechanism useful for this purpose.
Another problem exists when the heated fuser roll surface is contaminated by paper resin because of sustained copy runs. The heated fuser roll becomes sticky and therefore tends to offset and/or wrap on a heavily toned copy. Early roll closure is a known effective method of minimizing this problem for a fuser nip which is normally open. Early roll closure involves bringing the backup roll (whose temperature is, for example, on the order of 180.degree. F.) into circumferential engagement with the heated fuser roll (whose temperature is, for example, on the order of 355.degree. F.) prior to paper arriving at the fuser nip. This early closing of the nip allows the temperature of the heated fuser roll in the fuser nip to be reduced to a stable operating level by a brief period of conduction of heat to the backup roll. IBM TDB "Electromagnetically Latchable Damper Assembly" (7/80, pp. 474-475) describes an interruptable linkage associated with a heated fuser roll and a backup roll while the May 1981 IBM Technical Disclosure Bulletin in the article entitled "Fuser Configuration" (pp. 5622-5623 ) describes an arrangement in which a spring controls the closure force between the heated fuser roll and the backup roll.
Yet another problem associated with contemporary fuser rollers is wear of those rollers from operation under the relatively high closure pressure involved in fusing. Thus the useful life of such rollers is extended by minimizing the closure pressure level and/or time of closure when fusing is not performed.
The capability to override the contour of the cam and thus open the fuser nip during intersheet gaps, upon detection of paper jams or other emergency condition, and to facilitate maintenance is disclosed and claimed in copending application Ser. No. 145,084, filed Apr. 30, 1980.
Accordingly, it is the principal object of this invention to provide an improved xerographic toner fixing apparatus.
It is another object of this invention to provide an improved method and apparatus for early roll closure.
It is still another object of this invention to provide for an early roll closure which requires less closing force than that needed to fuse images to sheets.
A further object of this invention is to provide for improved heat conduction between the heated fuser roll and the backup roll.